1. Field of the Invention
The present invention relates to a magnetoresistance effect film for reading the magnetic field intensity of a magnetic recording medium or the like as a signal and, in particular, to a magnetoresistance effect film which is capable of reading a small magnetic field change as a greater electrical resistance change signal, and further relates to a magnetoresistance effect type head using such a magnetoresistance effect film.
2. Description of the Prior Art
Recently, there has been the development for increasing the sensitivity of magnetic sensors and increasing the density in magnetic recording and, following this, magnetoresistance effect type magnetic sensors (hereinafter referred to as MR sensors) and magnetoresistance effect type magnetic heads (hereinafter referred to as MR heads) using magnetoresistance change have been actively developed. Both MR sensors and MR heads are designed to read out external magnetic field signals on the basis of the variation in resistance of a reading sensor portion formed of magnetic material. The MR sensors have an advantage that a high sensitivity can be obtained and the MR heads have an advantage that a high output can be obtained upon reading out signals magnetically recorded in high density because the reproduced output does not depend on the relative speed of the sensors or heads to the recording medium.
However, conventional MR sensors which are formed of magnetic materials such as Ni.sub.80 Fe.sub.20 (Permalloy), NiCo or the like have a small resistance change ratio .DELTA.R/R which is about 1 to 3% at maximum, and thus these materials have insufficient sensitivity as the reading MR head materials for ultrahigh density recording of the order of several GBPSI (Giga Bits Per Square Inches) or more.
Attention has been recently paid to artificial lattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked, because their behavior is different from that of bulk metal. One of such artificial lattices is a magnetic multilayered film having ferromagnetic metal thin films and non-magnetic metal thin films alternately deposited on a substrate. Heretofore, magnetic multilayered films of an iron-chromium type, a cobalt-copper type and the like have been known. However, these artificial lattice magnetic multilayered films are not commercially applicable as they are because the external magnetic field at which a maximum resistance change occurs (operating magnetic field intensity), is as high as several tens of kilo-oersted.
Under these circumstances, a new structure which is called a spin valve has been proposed. In this structure, two NiFe layers are formed via a non-magnetic metal layer, and an FeMn layer is further formed so as to be adjacent to one of the NiFe layers.
In this case, since the FeMn layer and the NiFe layer adjacent thereto are directly exchange-coupled to each other, the direction of the magnetic spin of this NiFe layer is fixed in the range of several tens to several hundreds Oe in magnetic field intensity. On the other hand, the direction of the magnetic spin of the other NiFe layer is freely changeable by an external magnetic field. As a result, there can be achieved a magnetoresistance change ratio (MR ratio) of 2 to 5% in a small magnetic field range which corresponds to the degree of coercive force of the NiFe layer.
In the spin valve, by realizing a difference in relative angles of spins between two magnetic layers, the large MR change which differs from the conventional anisotropy magnetoresistance (AMR) effect is accomplished. This is realized by pinning of the magnetic layer spin due to the direct exchange coupling force between one of the magnetic layers and the antiferromagnetic layer. This exchange coupling can be the substance of the spin valve.
However, for putting the spin valve to practical use, there are various problems as described hereinbelow. The strength of the exchange coupling pinning the magnetic layer is represented by a magnitude of a unidirectional anisotropic magnetic field Hua which shifts. On the other hand, a temperature at which Hua is lost is set to be a blocking temperature Tb which represents a thermal stability. The generally used FeMn layers and other antiferromagnetic layers have the following problems with respect to the exchange coupling:
(1) The blocking temperature Tb is low, that is, in the range from 150 to 170.degree. C. As compared with the state of the bulk, the blocking temperature Tb is low so that an excellent thin film which can fully achieve an expected pinning effect can not be obtained. PA1 (2) Dispersion of the blocking temperatures Tb occurs. Specifically, because of a thin film, the film surface of the FeMn layer is composed of various crystal grains, and the individual crystal grains have their own blocking temperatures Tb. If all the crystal grains have the same blocking temperature Tb, no problem is raised. However, actually, some crystal grains have lower blocking temperatures Tb, while some crystal grains have higher blocking temperatures Tb. As a result, it is possible that there exist those grains with small exchange coupling which causes reversal of the spin, in the ferromagnetic layer at 80 to 120.degree. C. representing an operating temperature range on application (due to existence of crystal grains having lower blocking temperatures Tb). Then, the spin direction of the pinned ferromagnetic layer may be inclined as a whole so that the output voltage is lowered. Thus, it is desired that a high-quality antiferromagnetic thin film be provided wherein as many crystal grains as possible have the same high blocking temperature Tb.
For solving the foregoing problems, antiferromagnetic thin films made of Ru.sub.x M.sub.y Mn.sub.z (M represents at least one selected from Rh, Pt, Pd, Au, Ag and Re) have been proposed as preferred examples in Japanese Patent Applications Nos. 8-357608 and 9-219121.
On the other hand, as a material of a protective film which is stacked so as to be in abutment with the antiferromagnetic thin film, Ta is generally used while Cu or Hf is also used on occasion. As typical examples of the prior art, the followings can be cited.
(1) Journal of Magnetism and Magnetic Materials, 93 (1991) 101 (Dieny, IBM)
Information about a laminate structure represented by Si/Ta(50.ANG.)/NiFe(60.ANG.)/Cu(25.ANG.)/NiFe(40.ANG.)/FeMn(50.ANG.)/ Cu(50.ANG.) is disclosed. Herein, an antiferromagnetic layer is made of FeMn, and a protective layer is made of Cu and has a thickness of 50.ANG..
(2) Japan Journal of Applied Physics,
33 (1994) 1327 (Nakatani, Hitachi)
Information about a laminate structure represented by Si/Hf(50.ANG.)/NiFeCo(50.ANG.)/Cu(20.ANG.)/NiFeCo(50.ANG.)/FeMn(100.ANG.)/ Hf(50.ANG. ) is disclosed. Herein, an antiferromagnetic layer is made of FeMn, and a protective layer is made of Hf and has a thickness of 50.ANG..
(3) JP-A-9-147325
Information about a laminate structure represented by glass/Ta(100.ANG.)/NiFe(50.ANG.)/PtMn(200.ANG.)/Ta(100.ANG.) is disclosed. Herein, an antiferromagnetic layer is made of PtMn, and a protective layer is made of Ta and has a thickness of 100.ANG..
However, it has been experimentally found out that a very strict heat run test can not be cleared by merely using such conventional protective layer materials in combination with the antiferromagnetic thin films proposed in the foregoing Japanese patent applications. Specifically,
(1) After carrying out an unavoidable heat treatment (for example, at 250.degree. C. for 3 hours) in a manufacturing process, the MR ratio of a magnetoresistance effect film is lowered by about 30%. As a result, it is possible that sufficient outputs can not be ensured after completion as a spin valve head.
(2) It is possible that magnetization of a pinned ferromagnetic layer is rotated while keeping the temperature in the range of 120 to 140.degree. C. Specifically, if a spin valve head is incorporated into a high-rotation type disk drive where the operating temperature of the head increases to 120 to 140.degree. C., it is possible that the output is reduced with a lapse of time and finally, recorded information on a disk can not be read out.